Ammonia is synthesized today, more or less, by a single process arrangement with the bulk of research designed to improve the catalyst performance and reactor design. In recent years axial flow catalyst beds have been replaced by catalyst beds which use substantially radial or combined axial and radial flows to reduce pressure drop, thereby saving compression energy, and permitting smaller sized, higher activity catalyst to be used. Yet, radial flow based reactors also have some drawbacks associated with the top seal zone of the bed (commonly referred to as the freeboard zone). Drawbacks include regions of low-flow which can result in reduced catalyst effectiveness and localized "hot-spots," as well as a potential for fluidization of the freeboard zone caused by loss of catalyst seal height as the bed settles. Both radial and axial/radial designs require a seal zone of additional catalyst to circumvent excessive bypassing after the catalyst bed has settled.
Recent reactor developments are set forth in a series of Zardi et al. U.S. Pat Nos. 4,372,920; 4,405,562; 4,755,362; 4,769,220; 4,904,453; 4,952,375; and 4,963,338. In Zardi '920, one or more catalyst containing baskets enclosed in a cartridge module have a combination of perforate and imperforate walls said to permit both axial and radial flow through the beds.
In Zardi et al. '562, a heat exchanger arranged centrally and axially in the above cartridge is used to preheat the inlet gas by exchange of heat with the effluent gas. In addition, the preheated inlet gas is mixed with fresh gas for temperature control.
Zardi '362, '453 and '375 describe a procedure for retrofitting axial flow ammonia reactors into an axial/radial flow reactor based on the design described in the above patents.
Zardi '220 describes a heterogeneous synthesis converter containing a variable number of internal cartridges designed similarly to Zardi et al. '562 having several points at which fresh gas can be introduced.
Zardi '338 describes an allegedly improved process for heterogeneous synthesis where the synthesis catalyst is distributed in three catalytic beds, either an axial-radial combination or radial only, and control of the temperature is effected by fresh quench gas and indirect cooling with a central axial mounted exchanger.
U.S. Pat. Nos. 4,230,669 and 4,346,060 to Eagle et al. describe a radial ammonia converter system including first and second heat interchangers and first, second and third catalyst beds.